Date   : Tue, 25 Oct 2005 01:01:14 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Floppy pinouts and twists

It occurs to me that many readers won't have a mental picture of the
pin assignments and twisted floppy cables we've been discussing, so
here is a diagram:


Pin  BBC/SA400   IBM PC

2    (note 1)    Density

4    In Use      (unused)

6    DS3         (unused)

8    Index       Index


10   DS0 ------- Motor0 -.       .- Motor1 -- MotorOn  10
                         \      /
                          \    /
12   DS1 ------- DS1 -.  .--\--/---- DS0 --------- DS2  12
                      \/    \/
                      /\    /\
14   DS2 ------- DS0 -'  `- /--\---- DS1 --------- DS1  14
                          /    \
                         /      \
16   MotorOn --- Motor1 -'       `- Motor0 ------ DS0  16


18   Direction   Direction

20   Step        Step

22   Write Data  Write Data

24   Write Gate  Write Gate

26   Trk00       Trk00

28   Wr.Prot.    Wr.Prot.

30   Read Data   Read Data

32   Side 1      Side 1

34   Ready       Disk Changed


Pins 11, 13 and 15 are twisted too, but all odd-numbered pins are
ground connections so the twist leaves them unaffected.

Note 1: This is TG43, or "Track Greater Than 43" on some very old
drives/interfaces; used to indicate that the drive should reduce the
write current on the inner tracks.  Most drives do this automatically
and do not need this signal.

A drive configured for a PC will have no connection to pins 10 or 14,
but will have its drive select connected to pin 12 and its motor
control connected to pin 16.  Thus, when placed after the twist, it
will appear to a PC as Drive A, and when placed before the twist, it
will appear as Drive B.

To a more conventional controller circuit, the drive would appear as
Drive 1 (equivalent to B) when placed before the twist, but would not
work as Drive 0 when placed after the twist, because the conventional
controller would not activate DS2 and DS0 at the same time.  However,
if the drive were configured not to need the motor control signal, it
might work as Drive 2.

Some interfaces (including the BBC Micro) only implement DS0 and DS1,
not DS2 and DS3.  Some other interfaces only implement DS0...DS2,
leaving DS3 (pin 6) unconnected.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York